The present invention relates to a cell structure for a plurality of electrolyzer units and fuel cells which are electrically and geometrically connected in series, particularly for the evolution of hydrogen and oxygen or for operation with hydrogen and oxygen.
The characteristic elements of such cells are a central diaphragm, electrodes on both sides of the diaphragm, bipolar cell partitions and elastically flexible spacers between the electrodes and the cell partitions. (There is also, of course, a frame which is of less interest in the present context.) The conditions in the boundary layers of adjacent elements (that is, mechanical connections, electrical contacts, possible relative movements, fluidic transitions, starting points for corrosion, etc.) are of great significance for the electrochemical and mechanical characteristics of the cell and thus of the electrolyzer unit and of the fuel cell, because they affect in particular its efficiency and the service life. For the purpose of obtaining a structure which is mechanically or structurally as simple, sturdy and durable as possible, while the cell voltages for a given current density are as low as possible, a preference exists for the so-called zero gap arrangement, in which the electrodes rest directly against the diaphragm, without any gap and without any spacers.
As disclosed in German Patent Document DE-OS 32 24 556, the diaphragm may consist of a metal netting coated with oxide ceramics, which affords relatively good stability characteristics. Moreover, German Patent Document DE-PS 37 43 354 teaches that the electrodes may consist of porously coated, catalytically activatable perforated plates, metal nettings or expanded metal elements. Where such electrode units are combined with a metal-netting-supported diaphragm, the risk exists, among others, that, as a result of frictional events, local overloading or corrosive events, the metal netting in the diaphragm can become partially exposed, which may lead to a shorting of the electrodes.
The best solution for the foreseeable future for a zero gap configuration seems to be a combination of the electrodes with the diaphragm to form an integral unit (EDE-plate=Electrode-Diaphragm-Electrode plate), which permits the diaphragm to be constructed in an unsupported manner (that is, without any inner metal nettings, etc.). The integration of these three functional elements takes place during manufacturing. The diaphragm is made from a ceramic blank, onto which the electrodes are applied in powder or paste form, by means of sintering or reductive sintering in the manner known to those skilled in the art. German Patent Document DE-OS 32 24 555 and the related European Published Patent Applications 0 297 315 and 0 297 316, for example, address the manufacture of such EDE-units.
In such EDE-composites, the electrodes virtually take over the support of the interposed diaphragm. Moreover, in tests it has been found that such EDE-plates can provide high current densities at low cell voltages and moderate cell temperatures.
In the spaces required, among other things, for electrolyte storage, between the electrodes of the EDE-plate and the bipolar cell partitions (that is, in the anode and cathode spaces), electrically conductive spacers are arranged which contact and center the EDE-plate. Preferably, elastically flexible elements, such as corrugated sheets, sheet metal strips or metal wool are used. It is also possible to mold the spaces into the cell partitions; for example, as naps which project axially to both sides. Such a construction is disclosed, for example, in European Patent Document EP-OS 0 340 820, albeit not for a zero-gap arrangement, but rather for a micro-gap arrangement with a narrow distance between the electrodes and the diaphragm, which is produced by additional non-conductive spacers in the electrodes.
Another known electrode structure having a zero-gap arrangement, but without forming a unit with the diaphragm, is composed of a lamellar structure in the form of a plurality of straight sheet metal strips arranged vertically and perpendicular to the diaphragm, at a narrow distance from one another, with interposed spacers. The prestressed zigzagging spacers are also made of sheet metal strips, and are disposed between the straight sheet metal strips. The spacers are arranged such that their zigzagging front edges contact the diaphragm along the whole electrode height. The distance between the straight sheet metal strips is dimensioned such that, while the number of layers is as small as possible, a sufficient circulation of fluids, such as the electrolyte is still possible. Because of the spacer arrangement, the main delivery direction extends horizontally, and perpendicular to the diaphragm. However, because of the contacting conditions between the parts, a perpendicular flow of the electrolyte or of the generated or fed gas is also possible. In addition, the gas generated by electrolysis can rise substantially unimpaired in the anode or cathode space kept free by spacers between the electric and the cell partition.
A disadvantage of this type of electrode, however, lies in the necessary filigree, high-expenditure structure. Moreover, tests with such electrodes have indicated that they are inferior to other zero gap arrangements with porous electrodes, particularly of the EDE-unit, in that they require higher cell voltages to achieve a given current density. Another disadvantage is the relatively intensive substance transport due to the delivery effect (electrolyte circulation, gas transport) in the spaces between the lamellae.
It is therefore an object of the present invention to provide a cell structure for electrolyzer units and fuel cells having a zero-gap arrangement, which permits a still higher efficiency, more favorable operating conditions and a longer service life.
This object is achieved according to the invention, in which a zero-gap arrangement with a plate-shaped diaphragm and electrolyte-permeable electrodes on both sides, is combined with a lamellar structure which fills out the electrode space at least partially and rests against the electrode, the lamellar structure being designed in the manner of the described electrode. The lamellar structure according to the invention has several functions. It serves as a mechanical spacer; that is, as a spring-elastic support of the electrodes and the diaphragm, which provides an optimal introduction and distribution of force. It also forms an electrical connection between the electrode and the cell partition, with a favorable current distribution and contact; and it promotes electrolyte circulation and gas transport by means of its delivery effect.
Tests of electrolyzer units having such combination of the zero-gap arrangement and the lamellar structure have shown that, in comparison to known EDE-units with spacers, given current densities and gas generation rates are reached with still lower cell voltages and significantly lower cell temperatures. In other words, the invention reduces the electric or chemical energy requirement, and improves the operating conditions as a whole, which has a positive effect in many respects. In an embodiment without additional spacers, the bipolar cell partitions can be constructed in a particularly simple and low-cost manner as flat plates made of sheet metal.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.